Isaac - a serious storm - but no hurricane Katrina

A comparison of hurricane Katrina (2005) and tropical storm Isaac (2012)
(NOAA)
On the seven year anniversary of hurricane Katrina, to the day, Isaac is likely to produce hazardous if not dangerous tropical weather in New Orleans.

While residents of New Orleans should be taking Isaac very seriously and preparing for hurricane conditions, it’s unlikely to affect the area as severely.

Let’s briefly explore why Isaac is likely no Katrina ...

Satellite presentation

A day and a half before landfall, hurricane Katrina was a much more intense and mature tropical cyclone than tropical storm Isaac. That’s strikingly obvious when comparing satellite images of the two storms at the same juncture (above): Katrina is symmetric and has a well-defined eye surrounded by central dense overcast whereas Isaac lacks a clear eye and is somewhat asymmetric.

Wind and storm surge

Katrina reached high-end category 5 status, with maximum sustained winds to 175 mph at one point before they diminished to around 120 at landfall. Isaac’s maximum winds haven’t yet reached 70 mph though they may ramp up to 100 mph. Even so, Katrina is virtually assured to be the windier storm (and it’s also slightly bigger with tropical storm winds reaching a larger radius).

The stunningly low pressures reached within Katrina’s core coupled with its monstrous, destructive wind field, helped generate a devastating storm surge up to 27 feet in coastal Mississippi and 12-16 feet around New Orleans.

Isaac’s (slightly) smaller size, lower winds and higher pressures mean it should not generate a similar surge.

Isaac still hasn’t gotten its act together. Unlike Ike, which was producing storm surge all over the Gulf by the time it was in Isaac’s position, little storm surge is occurring from Isaac as of this writing.

For New Orleans, the angle at which Isaac approaches the coast will be important. If it makes landfall just to city’s west (worst case), it will produce a maximum storm surge into the city off the Gulf of Mexico. A track to the east (similar to Katrina) would still produce a substantial surge says AccuWeather but from a different direction:

Isaac is likely to be rolling in straight from the southeast, so the counterclockwise flow around the storm could drive a significant surge toward Chanderleur Sound, Lake Borgne and Lake Pontchartrain early on and for a number of hours.

Levees

A principal reason New Orleans is likely less vulnerable to Isaac than Katrina is due to the multi-billion dollar re-engineering of its levee system.

Some of the gates installed after Hurricane Katrina will be closing for the first time. Last year when Tropical Storm Lee hit the area, some of the gates were still under construction and were considered construction closures under the oversight of the Army Corps.

New Orleans Mayor Mitch Landrieu said at a news conference that “all pumps are operational”and “It’s absolutely true that we are better prepared.”

Rainfall

Katrina produced about 8-10” of rain in southeast Louisiana. This is one category where Isaac has the potential to outperform its 2005 cousin. Some computer models show the storm slowing down around the time of landfall, meaning heavy rain and wind battering the area an extended period of time. At least 8-12” is a good bet over a wide area from Isaac and some 20” totals are possible. Inland flooding may well be Isaac’s most hazardous and serious impact.

Conclusions

Isaac is simply not the storm Katrina was, but its impact could still be quite large due to the fact it could intensify prior to landfall (whereas Katrina was in a weakening phase). Also, Isaac’s forward motion may slow around landfall extending the timeframe the region experiences hurricane conditions relative to Katrina. But, due to the region’s better overall preparedness and the storm’s lesser size and intensity, Isaac should not pack the same overall punch.

(Note: before concluding, I should re-emphasize the importance of preparedness for those along the northern Gulf coast and in New Orleans. Forecasting hurricane intensity is still not very accurate and should Isaac unexpectedly rapidly intensify over the next 24 hours, the consequences could still be devastating)

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